Intracellular pathogens from viruses to eukaryotes cause significant disease worldwide. An essential process in the life cycle of these pathogens is the ability to efficiently exit from host cells after intracelular replication. As opposed to the wll-characterized strategies of exit such as budding or apoptosis, this proposal will focus on how intracellular parasites utilize cytolysis as an exit strategy, the molecular details of which are poorly understood. Our prior studies led to the discovery that these two apicomplexans both use the calcium regulated host protease calpain in order to exit from their host cells. We hypothesize that intracellular cytolytic parasites utilize a complex set of calcium regulated host proteins for efficient egress. We will test this hypothesis using two Apicomplexan organisms, P. falciparum and T. gondii. Plasmodium sp., which causes malaria, is responsible for worldwide morbidity and mortality for which new therapies are urgently needed. Toxoplasma infection poses serious problems during congenital infection of humans, agriculturally important animals, and in immunocompromised adults. Plasmodium and Toxoplasma are obligate intracellular pathogens with similar virulent lytic cycles. During these lytic cycles parasites invade host cells and establish a specialized compartment within the host cell called the parasitophorous vacuole, within which they complete their entire intracellular cycle. After replication, daughter parasite cells must exit their host cell via lysis of both the vacuolar membrane and the host plasma membrane. We herein propose to integrate cell biological, molecular genetics, and pharmacological approaches to elucidate the activation, function, and significance of this host network for parasite cytolysis.